Caloric restriction, in which the calories are restricted to maintain a stable adult body weight but all other nutritional needs are met, is the only proven intervention to extend life span and improve health. Calorically restricted rodents are used to investigate the cellular and physiological biology of normal aging. A colony of aged, calorically restricted laboratory rodents, of defined genetic backgrounds and health status, is an integral part of the NIA's program of research on aging. The purpose of this project is to develop, maintain and distribute a standing colony of aged, calorically restricted roents of defined strains for use by investigators in studies of aging.